Donald Trump told Fox News’ Greta Van Susteren Thursday night that he believes Sen. Ted Cruz should get a declaratory judgment from a federal court, ruling that Cruz is a natural born citizen eligible to run for president. What Trump may not know—but which his lawyers should have told him—is that Cruz can only get such a ruling if Trump sues him first.

Breitbart News has thoroughly analyzed the legal issues raised by Trump’s questioning whether Cruz is eligible under Article II of the Constitution to run for president. If Trump sues Cruz now, this matter could be decided by a federal court before Iowans go to their caucuses on February 1.

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When George Washington was elected as America’s first president, he had a legal question regarding America’s treaty with Great Britain and sent that question to the Supreme Court for the justices to rule on the matter. In 1793, the Supreme Court responded with a letter to Washington, including, “We exceedingly regret every Event that may cause Embarrassment to your administration,” but then explaining that the U.S. Constitution only gives federal courts the power to declare a legal interpretation when a proper lawsuit is brought before the courts.

From 1793 to 2016, the federal courts have been explicit that they cannot issue “advisory opinions.” Article III of the Constitution says the “judicial Power” only extends to certain types of cases. One of Article III’s requirements is adversity: a court can only declare legal decisions when there are at least two adverse parties—a plaintiff and a defendant—who are trying to defeat each other.

Otherwise, people could game the court system to twist and pervert the law. A plaintiff and defendant could conspire together, with one side making great arguments and the other party deliberately filing only pathetic arguments, trying to push the court in one particular direction. It is the equivalent of a boxing match where one fighter deliberately throws the fight to the other side.

As the Supreme Court has explained many times, requiring parties to be adverse means that a federal judge will get the best possible legal briefs and oral arguments that both sides can throw at each other. This gives the judge all the tools he needs to decide the case correctly.

Long ago, Congress passed the Declaratory Judgment Act, found at 28 U.S.C. § 2201, which empowers federal courts to “declare the legal rights and other legal relations” of parties to a lawsuit. But Congress can never exceed the powers granted to it by the Constitution, and the Constitution still requires both parties in a declaratory judgment lawsuit to be adverse.

(Many newsworthy lawsuits are declaratory judgments. Usually when a plaintiff asks a court to strike down a law, it is asking for a declaratory judgment. Most of the biggest lawsuits—such as the Obamacare case NFIB v. Sebelius, the gay-marriage case Obergefell v. Hodges, the Second Amendment case D.C. v. Heller, and the religious-liberty case Town of Greece v. Galloway—were all declaratory judgment lawsuits.)

Now that Trump has raised these issues, the whole country—not just the Republican Party—will benefit from a prompt resolution of the controversy. If Trump is right, then Republicans should know while they still have time to decide who the party’s nominee should be. Otherwise, Hillary Clinton could make this a political issue but never take it to court, in a cynical effort to depress conservative turnout at the polls and elect her as a liberal Democratic president.

On the other hand, if Trump is wrong, and Cruz is constitutionally eligible—as Breitbart News has already reported that both conservative and liberal legal scholars seem to think—then Republicans’ judgment in choosing their nominee should be informed by that knowledge, as well.

The only reason for Trump not to sue is if he intended this issue to cast doubt upon Cruz to help Trump win the Republican primaries, but believed that the law was actually on Cruz’s side. If so, he could continue to erode Cruz’s vote share by warning voters that the Texas senator would be an unconstitutional president, but never take it to court to be proven wrong.

In fact, if Trump is right that Cruz was not born a U.S. citizen, then even now, Cruz would not be an American citizen at all because he never went through the naturalization process required for all foreign immigrants, and would, therefore, not be eligible to be a U.S. senator. (Article I of the Constitution requires that all senators be U.S. citizens for at least nine years before being elected.)

If Trump moves for a preliminary injunction asking for the U.S. district court in Iowa to block Cruz from the Iowa caucuses because Cruz is ineligible, a federal judge would schedule briefing and oral argument within days. A court decision could be rendered within a week. The losing party (Trump or Cruz) could then file an emergency appeal to the U.S. Court of Appeals for the Eighth Circuit in St. Louis, which would likely schedule the case to be heard only days later and render a decision before the end of January.

The losing party at the appeals court could then petition the U.S. Supreme Court for review. If the Court chooses to intervene, it could likewise resolve the matter with rapid speed.

But the federal courts are powerless under Article III to act until someone with standing to sue files a declaratory-judgment lawsuit against Cruz. The clearest standing is the standing of a rival GOP candidate. If Trump believes Cruz’s eligibility is in doubt and that Cruz should get a declaratory judgment to settle the matter, then Trump should sue now. Either way, he would do himself, Cruz, and the country a favor by doing so.

Ken Klukowski is legal editor for Breitbart News. Follow him on Twitter @kenklukowski.